to determine the binary size of a single font, you can place the font into a separate unit and analyze the map file generated by your compiler/linker.
In order to reduce the size of a font, you can define a list of character codes that are used within your GUI project. Please have a look to the description of the attribute Ranges. The list of character codes can be created comfortable by using the Excel sheet template (see example StringTable). The macro is just in case you want to reduce the font range to the absolute minimum.
However, in case your application will display strings dynamically, you have to provide a larger font range. Especially in case of Chinese, Japanes or Korean language support, the resulting bitmap font will require several MByte of Flash memory.
In case your application has to provide thousands of characters and you want to use different font sizes, it might be more efficient to use a TrueType font engine within the target system instead of compressed bitmap fonts.
Maybe you can explain your use-case / your application more detailed to give you a recommendation.